package jjn.carl.monotonic_stack;

import java.util.Scanner;

/**
 * @author Jiang Jining
 * @since 2023-08-27 10:06
 */
public class LeetCode42 {
    public int trap(int[] height) {
        int length = height.length;
        if (length <= 2) return 0;
        int[] maxLeft = new int[length];
        int[] maxRight = new int[length];
        
        // 记录每个柱子左边柱子最大高度
        maxLeft[0] = height[0];
        for (int i = 1; i < length; i++) maxLeft[i] = Math.max(height[i], maxLeft[i - 1]);
        
        // 记录每个柱子右边柱子最大高度
        maxRight[length - 1] = height[length - 1];
        for (int i = length - 2; i >= 0; i--) maxRight[i] = Math.max(height[i], maxRight[i + 1]);
        
        // 求和
        int sum = 0;
        for (int i = 0; i < length; i++) {
            int count = Math.min(maxLeft[i], maxRight[i]) - height[i];
            if (count > 0) sum += count;
        }
        return sum;
    }
    
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int total = scanner.nextInt();
        int[] height = new int[total];
        for (int i = 0; i < total; i++) {
            height[i] = scanner.nextInt();
        }
        int trap = new LeetCode42().trap(height);
        System.out.println(trap);
    }
}
